In a 3-qt. saucepan over medium heat, combine water, bouillon, potatoes, onion, celery, salt and pepper; bring to a boil. Reduce heat cover and simmer for 15-20 minutes or until potatoes are tender. Add 1-3/4 cups milk. Combine flour with remaining milk; stir to form a smooth paste. Add to soup, stirring constantly. Bring to a boil; boil and stir for 2 minutes or until thickened and bubbly. Add a small amount of hot liquid to sour cream; stir to mix. Gradually add to soup, stirring constantly; heat through but do not boil. Add parsley and chives just before serving.